With open standards, there are no expensive lock-ins, no expensive upgrades and updates. There will be a need to update your system with your post-edited material, yes, but this is the learning curve of the system. An engine pays for itself in saved translation fees before one year. An update with post-editing material is a fraction of that cost.
Once your development reaches maturity, there will be little need for maintenance -unless you are a heavy duty corporate user with very specific requirements. You can then concentrate on producing more and more translated material, or consider the experience to create more customized engines.